Tuesday, October 13, 2009

Willow Pine Cones

What's a pine cone doing on a willow?

It's not really a pine cone.

It's a gall, an abnormal cell growth caused by chemicals either injected or secreted by an insect or mite, a nematode or disease agent usually during a period of rapid plant growth (late spring).  These chemicals cause increased production of normal plant growth hormones which result in hypertrophy (increased cell size) and/or hyperplasia (an increased number of cells).  When the gallmaker is an insect, the gall provides food and shelter for the developing offspring.

Galls are unique to the plant and the gall-maker.

Several distinctive galls can be found on willows:  the willow blister gall, the fleshy willow leaf gall and the willow beaked gall.

In this case, the gall maker is a tiny dipteran - a midge known as Rhabdophaga strobiloides.  She lays a single egg in the terminal buds of willows.  When the egg hatches, the bud stops growing its typical leave.  Instead, the growth of flatten scales looks like a pine cone.  The larvae (maggot) over-winters in the gall and emerges as an adult in the spring.

An interesting aside about the entomologist who named this species,  Baron Karl Robert von Osten-Sacken.   A Russian diplomat living in the United States during the Civil War, Osten-Sacken published many papers on Diptera and was known for his work on insect galls.

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